As a method for preparing a steel pipe with a relatively small diameter from a steel strip, two processes are known including a solid phase welding pipe-making process (i.e. a solid phase pressure-welding pipe-making process) such as a butt-welding process wherein an open pipe formed by continuously forming a steel strip in the form of a pipe is entirely heated to high temperatures and is pressure-welded at both edges thereof, and a welding pipe-making process wherein an open pipe is welded at both edges thereof such as by electric resistance welding, laser welding or the like.
The solid phase welding process is usually adapted for mass production of small diameter pipes with an outer diameter of 115 mm or below. However, this process is disadvantageous in that since the open pipe is heated to high temperatures from the outer peripheries thereof, a scale loss becomes so great that the resultant product becomes poor in surface texture. On the other hand, with the welding process, only both edges of the open pipe are heated to temperatures higher than the melting point at the time of the welding. The portions other than the edges are in a cold condition of 100.degree. C. or below. Thus, the problem of the surface roughening as experienced in the solid phase welding process does not arise. However, this process is a cold process, so that it is necessary to prevent the occurrence of slip defects as will be caused between pipe-making tools, such as a caliber roll, and the open pipe, and to take a measure for suppressing a forming load. Thus, the production efficiency becomes poor. In addition, because the use of caliber rolls which are in conformity with the dimension of a product steel pipe is essential, the welding process is not suited for the small lot and multikind manufacture of steel pipes.
In order to overcome the disadvantages involved in the steel pipe-making method using the solid phase butt-welding process or the welding process, methods of the cold reducing of a steel pipe by welding processes have been proposed as disclosed such as in Japanese Patent Unexamined Publication Nos. 63-33105 and 2-187214.
When, however, a steel pipe obtained by a welding process is subjected to the cold reduction, a great rolling load is required. This, in turn, inevitably requires the installation of a lubricating rolling device for preventing galling or seizing defects with the roll, or the installation of a large-scale mill which can stand use under the great rolling load. Moreover, when a steel strip is formed into a stock pipe (i.e. an open pipe), the strain of the forming is established, to which the work strain caused during the course of the cold reduction is added. Hence, the steel suffers a considerable degree of the work strain, with the attendant problem that after the making of the pipe, a thermal treatment step has to be added.
Further, as disclosed in Japanese Patent Examined Publication No. 2-24606 and Japanese Patent Unexamined Publication No. 60-15082, there have been proposed methods where a steel pipe obtained by a welding process is hot reduced.
However, after the steel pipe formed by this welding process has been hot reduced, the mother pipe is again heated to 800.degree. C. or above in a reheating furnace. The brings about a fresh scale loss, coupled with another problem that when reduced, scale inclusion is induced.
An object of the invention is to solve the problems of the prior art and to provide a method and apparatus for reducing a steel pipe wherein a steel mother pipe prepared according to a solid phase joint or welding process or a welding process is reducible at low load and while suppressing work hardening without worsening the surface properties and wherein the dimensional accuracy of a product steel pipe can be maintained at a high level.